The scanning electron microscope/electron probe microanalyzer described in this proposal will provide the capability to analyze a variety of biomaterials, including metals, polymers, ceramics and biological tissues. It will serve as a replacement for an aging electron microprobe which has been at the focal point of numerous significant discoveries in dental materials during the past twenty years, several of which have been directly responsible for modifications in the composition of dental amalgam. These discoveries have advanced restorative materials. In addition, the instrument will provide the SEM capabilities that are presently lacking. One of the projects in which the SEM will be used is involved with delineating the factors leading to the premature failure of current dental composite restoratives. This study will attempt to evaluate various in vitro experiments in terms of their ability to discriminate between adequate and inadequate formulations. The second study employing the SEM and the x-ray analytical capabilities of the microprobe is concerned with documenting and correlating the changes in morphology and calcium levels in the lens of the eye during cataract formation. This experiment is a portion of a proposal designed to better understand the biochemical mechanism involved in selenium-induced cataracts. The electron microprobe capabilities will be further used to better define the correlation between the in vitro properties and the in vivo behavior of dental amalgam. Microstructural and compositional characterizations of various amalgams will be performed to achieve this goal. In addition, the electron microprobe will be used in another study involved with quantifying and identifying the various mechanisms of mercury release from dental amalgam. In answer to the recent concern over the use of mercury in dentistry, this proposal aims to determine maximum mercury exposure levels for patients and dental personnel. The use of the instrument proposed will be supervised by an electron microprobe operator having twenty years of practical experience. The instrument fulfills the requirements outlined in four current NIH sponsored projects, and can be justified on the basis of updating currently available equipment as well as providing new capabilities not readily available at our institution.